The invention relates to a nickel-cadmium storage battery with improved electrical properties, in which the active material with which the negative electrode is filled or the electrolyte in the storage battery contains polyvinylpyrrolidone as expander.
The cadmium electrode is used as a cathode almost exclusively in nickel-cadmium storage batteries. The nickel oxide electrode serves as positive electrode and an aqueous KOH solution as electrolyte. It has long been known that, like other negative electrodes, the cadmium electrode yields an adequate capacity during discharge only as a result of adding so called expanders (also call extenders). Without the addition of such expanders, a rapid drop in the capacity of the cadmium electrode occurs during cyclic charging and discharging, due to a coarsening of the crystals of the solid reactants Cd or Cd(OH).sub.2. The expanders counteract this process. That is, they keep the active constituents of the cadmium electrodes in a finely divided state.
Inorganic substances which, like Fe, Fe.sub.2 O.sub.3, Ni or Ni(OH).sub.2, for example, have long been used extensively as expanders. Iron oxide is added to the cadmium active material, and iron or nickel is deposited together with the cadmium active material by simultaneous electrolysis. The nickel hydroxide can reach the cadmium electrode in several ways. In sintered and sintered foil electrodes it is produced by corrosion of the nickel framework. It can be deliberately introduced by impregnation with a nickel salt solution and subsequent precipitation with alkali (East German Patent Specification 12053). Furthermore, it can be added to the active material as nickel hydroxide powder (U.S. Pat. No. 3,870,562 and German Patent Specification 2,445,096) or introduced in the form of a suspension (V. A. Reshetov et al., "Journal Appl. Chem. USSR", 52 (1979), pages 548 ff., page 551). Some gastight nickel-cadmium storage batteries contain nickel hydroxide as an antipolar mass in the cadmium electrodes.
These additives have several known disadvantages. The iron and the iron oxides or hydroxides result in so-called iron poisoning of the positive electrode and, consequently, in a decrease in this capacity. In addition, the proportion, normally used (10 to 40% by weight of iron) reduces the capacity per unit weight of the cadmium electrode.
With nickel or nickel compounds, cadmium forms the intermetallic phase Ni.sub.5 Cd.sub.21 which is discharged at a potential about 150 mV more anodic that cadmium. The extent of the phase formation depends on various parameters, such as temperature, electrolyte concentration and a particle size of the nickel or the nickel hydroxide. The amount of the phase formed increases with increasing temperature and decreasing particle size of the nickel or the nickel compound. Since more than four atoms of cadmium are bound, i.e. inactivated, per atom of nickel, the addition of larger amounts of nickel, nickel hydroxide or other nickel compounds is ruled out.
East german Patent Specification 12,053 describes the addition of 1% by weight of MgO or Mg(OH).sub.2 to the active cadmium material, which is also activated in addition with nickel sulphate. British Patent Specification 792,464 mentions, in addition to known substances, the addition of copper, copper oxide, cobalt, cobalt oxide, mercury and/or mercury oxide. A. Fleishcer, "Proc. 11th Ann. Battery Research and Development Conference" (1957), pages 83 ff., reports on the positive effects of indium on the capacity of the cadmium electrode, and O. C. Wagner, "J. Electrochem. Soc.", 116 (1969), pages 693 ff., reports on the use of 5-10% by weight of TiO.sub.2 instead of Fe.sub.2 O.sub.3 in cadmium electrodes.
Certain organic compounds have proved more suitable as expanders than the above substances because they need be added in amounts of only up to about 1% by weight to the active cadmium material. According to the teaching of U.S. Pat. No. 2,802,043, alkali-soluble cellulose derivatives--particularly carboxymethylcellulose (CMC)--are suitable. Associated with the electrolyte solubility is, however, a disadvantage, the CMC being oxidized at the positive electrode. With increasing operating time, the CMC disappears, and instead, carbonate accumulates as an oxidation product in the electrolyte, which has a known adverse effect on the capacity of the cadmium electrode.
Polyvinyl alcohol, which is used fairly often as an additive to cadmium electrodes, must be regarded more as a binder than as an expander. According to the teaching of U.S. Pat. No. 4,471,038, the addition of polybenzimidazole, polybenzothiazole and polybenzoxazole to the electrolyte or to the cadmium electrode of Ni/Cd storage batteries is beneficial for increasing the capacity or the cadmium utilization of the electrode. S. Sathyanarayana "J. Appl. Elektrochem.", 15 (1985), pages 453 ff., reports the unusually high cadmium utilization of 96.+-.2% on addition ethylcellulose to sintered nickel electrodes. According to our investigations, however, the ethylcellulose does not have any expanding action.
Ceratin pyrazolone derivatives were investigated by M. G. Mikhalenko et al., J. Appl. Chem. USSR", 49 (1976), pages 2022 ff. as expanding additives. A disadvantage cited is that, in the initial cycles, the cathodic reduction of CdO partly takes place only in the overcharging phase during the hydrogen evolution. The electrodes are compared with those which contain "solar oil" as expanding additive. P. V. Vasudeva Rao et al., "J. Power Sources", 1 (1976), pages 81 ff., reports on increased cadmium utilization on adding tetraalkylammonium compounds, particularly at discharge rates between 1.degree. C. and 10.degree. C.
According to the teaching of U.S. Pat. No. 2,870,234 and German Patent Specification 1,063,665, polyvinylpyrrolidone is suitable as an expander in cadmium electrodes. An addition of about 1% by weight, based on the active cadmium material, is specified. When the active cadmium material specified in these documents is used for the negative electrodes of alkaline nickel-cadmium storage batteries, however, it has been found that the storage batteries sometimes malfunction, and there is not an adequate cadmium utilization.
The object of the present invention, therefore, is to provide an alkaline nickel-cadmium storage battery, in which polyvinylpyrrolidones having a particular characteristic are added as an expander to the active material with which the negative electrode is filled or to the electrolyte in the storage battery, so that the above-mentioned disadvantages no longer occur.
Another object of the invention is to provide a storage battery that has a good charge acceptance, with a low dosage of added polyvinylpyrrolidone in terms of weight.
These and other objects and advantages are achieved, according to the invention, by the addition of the polyvinylpyrrolidone compound, either to the aqueous active material paste, or as an aqueous solution to the electrolyte.